The present invention relates to a method of manufacturing an inner or outer ring for a rolling bearing and, in particular, to such a method which manufactures an inner or outer ring for a rolling bearing by working the inner or outer ring plastically.
Conventionally, when an inner or outer ring for a rolling bearing is manufactured by plastic working, after a cylindrical steel billet is held and pressed by and between upper and lower metallic molds, that is, the steel billet is upset, the central portion of the cylindrical steel billet in the axial direction thereof is pressed, that is, the steel billet is centrally pressed to thereby reduce the thickness of the central portion thereof, then the inner portion of the centrally pressed portion of the steel billet is removed by punching or the like to thereby produce a ring-shaped workpiece, after then the outer peripheral surface of the ring-shaped workpiece is held by a forming roll and is also rolled by use of a mandrel roll, and the ring-shaped workpiece is then shaved by machining to thereby manufacture an inner or outer ring for a rolling bearing.
However, in the above-mentioned conventional inner ring manufacturing method, due to the fact that, as described above, in the centrally pressing step after the upsetting step, the axially central portion of the cylindrical steel billet is pressed only from above, the direction of the metal fiber flows which will be hereinafter referred to simply as "metal flows") of the steel billet cannot be identical with the shape of the race surface of the inner ring. Therefore, when the race surface is formed by machining, the metal flows are cut off and the cut-off surfaces are exposed to the race surface, which causes the life of the rolling bearing to be shortened.
In addition, in the above-mentioned conventional rolling bearing outer ring manufacturing method, while the cylindrical steel billet is heated before it is upset, because the lower metallic mold has a longer contact time for contact with the heated steel billet than the upper metallic mold, a temperature distribution is not uniform but the temperature becomes lower as it goes down toward the bottom. Therefore, when an upsetting step is carried out once as in the above conventional manufacturing method, as shown in FIG. 6, metal fiber flows (which will be hereinafter referred to simply as "metal flows") 51 become asymmetric with respect to the vertical direction thereof. Also, the center pushing is executed from one direction, which increases the asymmetry of the metal flows 51. As a result of this, when the rolling operation is finally executed, the metal flows 51 of the workpiece are not identical with the race surface of a bearing outer ring or race. Accordingly, when the workpiece is machined and formed, the metal flows 51 are cut off and the cut-off surface is exposed onto the outer ring surface, which causes the life of the bearing to be shortened.